1. Technical Field of the Invention
The present invention relates generally to flow control devices and, more particularly, to a uniquely configured control valve which is adapted to provide velocity control trim in a short stroke to accommodate space or design constraints.
2. Description of the Related Art
There is currently known in the prior art control valves which include stems and plugs that are linearly displaced during normal operation of the valve. Within these valves, which are often referred to as linear displacement valves, the plug is disposed and moveable within a disk stack or valve cage which defines a multiplicity of tortuous and/or non-tortuous fluid passageways. Certain linear displacement valves are configured for “over plug flow” wherein fluid flows radially inward into the interior of the valve cage from the exterior thereof, with the fluid undergoing a pressure drop as a result of the flow through the passageways of the valve cage. In this arrangement, the valve is opened by lifting the plug off a seat ring which thus allows the fluid to flow from the interior of the valve cage and out of the valve via the unblocked seat ring. Conversely, movement of the seating surface of the plug into sealed engagement with the complimentary seating surface of the seat ring facilitates a closed or shut-off condition for the valve.
As an alternative to over plug flow, other linear displacement valves are configured for “under plug flow” wherein fluid flows axially upwardly into the interior of the valve cage from the exterior thereof, with the fluid undergoing a pressure drop as a result of the flow of the fluid radially outwardly through the fluid passageways of the valve cage. In this arrangement, the valve is opened by lifting the plug off of the aforementioned seat ring, which thus allows the fluid to flow into the interior of the valve cage and thereafter radially outwardly through the fluid passageways of the valve cage. Conversely, the movement of the seating surface of the plug into sealed engagement with the complimentary seating surface of the seat ring facilitates a closed or shut-off condition for the valve.
Linear displacement control valves are often used to control flow and pressure in a process. However, in some applications and valve designs, there is a limitation in the maximum allowable stroke due to space or design constraints. Such design constraints may be attributable to, for example, the use of bellows seals to prevent leakage through the stem seal in valves used in conjunction with toxic or radioactive process fluid. Thus, there exists a need in the art for a linear displacement control valve which is capable of providing a velocity control trim in a short stroke. The present invention addresses this particular need by providing a short stroke control valve which provides the same velocity control capacity as a conventional normal stroke control valve. These, as well as other features and advantages of the present invention will be described in more detail below.